The present invention relates to a method for manufacturing a preform for optical fibres by means of a vapour deposition process.
A device for manufacturing a preform for optical fibres by means of a vapour deposition process is known per se from Korean patent application No. 2003-774,952. Using the device that is known therefrom, an optical preform is made by means of an MCVD (Modified Chemical Vapour Deposition) process, in which a discharge tube and an insert tube are used, which discharge tube is attached to the substrate tube. The insert tube is disposed within the discharge tube and has an external diameter which is smaller than that of the discharge tube. Disposed within the insert tube is a soot scraping element, which comprises a bar which rotates in the interior of the insert tube and which is in contact with the inner surface thereof. Between the insert tube and the discharge tube is an annular space, through which gases are passed.
From International application WO 89/02419 there is known a device for manufacturing an optical preform by means of an internal vapour deposition process, wherein a tubular element is mounted at the pump side of a substrate for the removal of solid, undeposited particles. In particular, such a device comprises a screw structure which follows the inner surface of the tubular element, which screw structure comprises an open gas conduit which is spiral-shaped and rotatable.
During the deposition of doped or undoped glass layers on the interior of a substrate tube, in particular by means of the PCVD (Plasma Chemical Vapour Deposition) process, low-quality quartz layers may be deposited, in particular in the area outside the range of movement of the energy source being reciprocated along the length of the substrate tube, viz. the resonator. Examples of such a low-quality quartz layers are so-called soot rings, but also but also quartz having a high internal stress caused by a high dopant content.
The present inventors have found that such low quality quartz present in the interior of the substrate tube may adversely affect the substrate tube, in particular as a result of the formation of gas bubbles in the vicinity of the supply side of the substrate tube upon collapsing the hollow substrate tube into a solid preform. In addition to that, the present inventors have found that such low quality quartz may come loose from the interior of the hollow substrate tube during the collapsing process, which may lead to contamination or to the formation of gas bubbles elsewhere in the substrate tube. Another negative aspect is the fact that cracks may occur in the low quality quartz region, which cracks may propagate in the direction of the center of the substrate tube, which is undesirable.
The present inventors have furthermore found that the low quality quartz may lead to clogging of the substrate tube and the associated piping, as a result of which the pressure may increase to an undesirably high value during the deposition process, which will have an adverse effect the deposition process in the substrate tube, which effect will be perceived as a white color in practice.
The substrate tube itself is made of high-quality quartz. In practice, however, the total length of the substrate tube will be greater than the length of the part of the substrate tube that is eventually converted into a glass fibre by means of a drawing process, because the two ends of the substrate tube, where deposition takes place, may cause undesirable side effects, viz. deposition defects, contamination, formation of bubbles and the like.
The present inventors have in particular found that, in the situation in which a so-called insert tube is used on the discharge side of the hollow substrate tube, the deposition of soot in particular occurs at high deposition rates, notably deposition processes in which the deposition of glass layers takes place at a rate of more than 3 g/minute, which processes generally last longer than 5 hours. Such an insert tube has an external diameter which is smaller than the internal diameter of the hollow substrate tube itself, and generally it is disposed within the substrate tube with a close fit, near the discharge side thereof. As a result of the buildup of soot in the insert tube, the pressure within the hollow substrate tube will increase, so that the deposition efficiency of SiCl4 will decrease further and (even) more soot will be formed, which is undesirable. Such a process is self-reinforcing, which means that the deposition process will have to be terminated due to a complete blockage of the passage of the insert tube on the discharge side of the hollow substrate tube. The interruption of the deposition process is considered to be undesirable.